1. Field of the Invention
The present invention relates to the field of surface defect detection, and more particularly relates to the field of defect detection in planar surfaces, especially the surfaces of optical discs and related products.
2. Background Art
The development of optical discs and other high density recording media used for the recording and playback of video information has given rise to a need for very accurate defect detecting methods and apparatus. The optical disc, for example, contains stored information on the disc surface in the form of spiral tracks of optically readable indicia having dimensions and spacings of the order of approximately one micron. Defects in the surface of an optical disc, for example, tiny bumps or pits of even microscopic dimensions, can cause the interruption of signal recovery. However, because of their extremely small size such defects can easily escape detection by an unaided observer.
Consequently, apparatus have been devised for detecting such microscopic defects. Typically, such apparatus utilize a narrow beam of light, for example, a beam of laser light, which is imaged to a small spot on the disc and scanned across the disc, and then some property of the light which is reflected or scattered from the disc is detected. According to one such scheme, a histogram is automatically generated which correlates the sizes and numbers of surface defects detected in the surface of an optical disc.
However, it is desirable to know the precise location of defects on an optical disc, or like surface, in order to determine the physical pattern of occurrence of defects. Toward this end, it has been proposed to provide a videodisc on a rapidly spinning turntable that is translated under an imaged beam of laser light, to detect diffraction components generated thereby, and couple to the turntable a second turntable upon which is mounted a disc of electric recording paper. As defects are detected by the laser beam apparatus, marks are recorded on the paper, thus producing a map of the defects. A writing pen is used to record the marks on the paper.
A high-powered microscope is focused onto the disc at the spot where the laser light impinges on the disc. A low-powered microscope is focused on the tip of the writing pen where it contacts the paper. After the map is made, the leading edge of a mark on the recording paper is positioned under the low-powered microscope, to automatically bring the coupled primary turntable under the high-powered microscope and allow the selective observation of areas of the disc in which defects have been detected. While this scheme does propose to provide a map of defects on a videodisc, the use of a coupled secondary turntable and writing pen, limits the speed and response time of the system. Indeed, the marks recorded on the recording paper by such an apparatus take the form of tangential lines of considerable length. Where it is desired to have a map which provides an accurate depiction of the density of occurrance of dropouts, such a scheme provides limited resolution.
It is therefore desired to have apparatus capable of generating a map of defects in a disc, such as an optical disc, wherein such apparatus has rapid response so as to enable the generation of a map which accurately depicts the density of occurrence of defects on the disc. The present invention satisfies this need.